Vaccine adjuvants such as alum and the oil-in-water emulsion MF59 are used to enhance immune responses towards pure soluble antigens, but their mechanism of action is still largely unclear. Since most adjuvanted vaccines are administered intramuscularly, we studied immune responses in the mouse muscle and found that both adjuvants were potent inducers of chemokine production and promoted rapid recruitment of CD11b(+) cells. The earliest and most abundantly recruited cell type are neutrophils, followed by monocytes, eosinophils and later dendritic cells (DCs) and macrophages. Using fluorescent forms of MF59 and ovalbumin (OVA) antigen, we show that all recruited cell types take up both adjuvant and antigen to transport them to the draining lymph nodes (LNs). There, we found antigen-positive neutrophils and monocytes within hours of injection, later followed by B cells and DCs. Compared to alum, MF59-injection lead to a more prominent neutrophil recruitment and a more efficient antigen re-localization from the injection site to the LN. As antigen-transporting neutrophils were observed in draining LNs, we asked whether these cells play an essential role in MF59-mediated adjuvanticity. However, antibody-mediated neutrophil ablation left MF59-adjuvanticity unaltered. Further studies will reveal whether other single cell types are crucial or whether the different recruited cell populations are redundant with overlapping functions.
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